belt, according to some astronomers, may be accounted for by supposing that the atmosphere reflects more light than the body of the planet, and that the clouds which float in it, being thrown into parallel strata by the rapidity of its diurnal motion, form regular interstices, through which are seen its opaque body, or any of the permanent spots which may come within the range of the opening.

Jupiter is also attended by four satellites or moons, some of which are visible to him every hour of the night; exhibiting, on a small scale and in short periods, most of the phenomena of the solar system. When viewed through a telescope, these satellites present a most interesting and beautiful appearance. The first satellite, or that nearest the planet, is 259,000 miles distant from its centre, and revolves around it in 42 hours; and appears, at the surface of Jupter, four times larger than our Moon does to us. His second satellite, being both smaller and farther distant, appears about the size of ours; the third, somewhat less; and the fourth, which is more than a million of miles from him, and takes 16 days to revolve around him, appears only about one third the diameter of our Moon.

These satellites suffer frequent eclipses from passing through Jupiter's shadow, in the same manner as our Moon is eclipsed in passing through the Earth's shadow. The three nearest satellites fall into his shadow, and are eclipsed, in every revolution; but the orbit of the fourth is so much inclined, that it passes by its opposition to him, two years in six, without falling into his shadow. By means of these eclipses, astronomers have not only discovered that light is 8 minutes and 13 seconds in coming to us from the Sun, but are also enabled to determine the longitude of places on the Earth with greater facility and exactness than by any other methods yet known.

It was long since found, by the most careful observations, that when the Earth is in that part of her orbit which is nearest to Jupiter, the eclipses appear to happen 8′ 13" sooner than the tables predict; and when in that part of her orbit which is farthest from him, 8′ 13′′ later than the tables predict; making a total difference in time, of 16′ 26′′. From the mean of 6000 eclipses observed by Delambre, this disagreement between observation and calculation, was satisfactorily settled at 8' 13", while both were considered_equally correct. Now when the eclipses happen sooner than the tables, Jupiter is at his nearest approach to the Earth-when later, at his greatest distance; so that the difference in his distances from the Earth, in the two cases, is the whole diameter of the Earth's orbit, or about 190 millions of miles. Hence, it is concluded that light is not instantane

How many satellites has Jupiter? How often are they visible to him? What is the distance from him of his first or nearest satellite? What is the time of its revolution ? What is its apparent magnitude at the surface of Jupiter, compared with the magnitude of the Moon, as seen oy us? What are the apparent magnitudes of his other satellites, as seen at his surface, compared with that of the Moon as seen at the Earth? What is the distance of his fourth satellite from him? What is the time of its revolution? How often are his three nearest satellites eclipsed? How often his fourth? Why is it not eclipsed as often as the others? What important purposes have these eclipses served to Bзtronomers? State the method by which the progressive motion of light, and the time which it occupies in coming to us from the Sun, were discovered.

ous, but that it occupies 16′26′′ in passing across the Earth's orbit, or 8′13′′ in coming from the Sun to the Earth; being nearly 12 millions of miles a

minute.

The revolutions of the satellites about Jupiter are precisely similar to the revolutions of the planets about the Sun. In this respect they are an epitome of the solar system, exhibiting, on a smaller scale, the various changes that take place among the planetary worlds.

Jupiter, when seen from his nearest satellite, appears a thousand times larger than our Moon does to us, exhibiting on a scale of inconceivable magnificence, the varying forms of a crescent, a half moon, a gibbous phase, and a full moon, every 42 hours.

The apparent diameters of Jupiter's satellites, their mean distances from him, and their periodical revolutions, are exhibited in the following table.

Saturn is situated between the orbits of Jupiter and Herschel, and is the most remote planet from the Earth of any that are visible to the naked eye. It may be easily distinguished from the fixed stars by its pale, feeble, and steady light. It resembles the star Fomalhaut, both in colour and size, differing from it only in the steadiness and uniformity of its light.

From the slowness of its motion in its orbit, the pupil, throughout the period of his whole life, may trace its apparent course among the stars, without any danger of mistake. Having once found when it enters a particular constellation, he may easily remember where he is to look for it in any subsequent year; because, at a mean rate, it is just 2 years in passing over a single sign or constellation.

Saturn's mean daily motion among the stars is only about 2', the thirtieth part of a degree.

Saturn entered the constellation Virgo about the beginning of 1833, and continued in it until the middle of the year 1835, when he passed into Li

In what respect are Jupiter's satellites an epitome of the solar system? What is Jupi ter's appearance, as seen from his nearest satellite? What are the diameters, mean dis tances, and times of the revolution of his satellites? Where, in the solar system, is Saturn situated? How may it be distinguished from the fixed stars? What star does it resemble? In what respects is it like it, and in what is it different from it? How may his place among the stars be readily found? What is about the rate of his mean daily motion among the stars? When did Saturn enter the constellation Virgo, and how long did he continue in it? What constellation did he enter next, and how long will he continue in it?

ora.

He will continue in that constellation until 1838; and so on; occu pying about 24 years in each constellation, or nearly 30 years in one revo. lution. The mean distance of Saturn from the Sun is nearly double that of Jupiter, being about 909 millions of miles. His diameter is about 82,000 miles; his volume therefore is eleven hundred times greater than the Earth's. Moving in his orbit at the rate of 22,000 miles an hour, he requires 291 years to complete his circuit around the Sun: but his diurnal rotation on his axis is accomplished in 101 hours. His year, therefore, is nearly thirty times as long as ours, while his day is shorter by more than one half. His year contains about 25,150 of its own days, which are equal to 10,759 of our days.

The surface of Saturn, like that of Jupiter, is diversified with belts and dark spots. Dr. Herschel sometimes perceived five belts on his surface; three of which were dark, and two bright. The dark belts have a yellowish tinge, and generally cover a broader zone of the planet than those of Jupiter.

To the inhabitants of Saturn, the Sun appears 90 times less than he appears to the Earth; and they receive from him only one ninetieth part as much light and heat. But it is computed that even the ninetieth part of the Sun's light exceeds the illuminating power of 3,000 full moons, which would be abundantly sufficient for all the purposes of life. Fig. 18.

The telescopic appearance of Saturn is unparalleled. It is even more interesting than Jupiter, with all his moons and belts. That which eminently distinguishes this planet from every other in the system, is a magnificent zone or ring, encircling it with perpetual light.

The light of the ring is more brilliant than the pla

How long time does he occupy in passing through each constellation, and what is the length of his year? What is his distance from the Sun? How much greater is this than Jupiter's distance? What is his diameter? How much greater is his volume than that of the Earth? What is the rate per hour of his motion in his orbit? In what time is his diurnal motion on his axis performed? How many of his own days does his year contain, and how many of ours? What is the appearance of his surface to us? How many belts did Dr. Herschel perceive on his surface? Describe them. How much less does the Sun appear to the inhabitants of Saturn than to us? What degree of light and heat does he receive from the Sun, compared with that received by the Earth? To the light of how many full moons is this degree of light equal? Describe the telescovic appearance of Saturn?

net itself. It turns around its centre of motion in the same time that Saturn turns on its axis. When viewed with a good telescope, it is found to consist of two concentric rings, divided by a dark band.

By the laws of mechanics, it is impossible that the body of the rings should retain its position by the adhesion of the particles alone; it must ne cessarily revolve with a velocity that will generate centrifugal force suffi. cient to balance the attraction of Saturn. Observation confirms the truth of these principles, showing that the rings rotate about the planet in 10 hours, which is considerably less than the time a satellite would take to revolve about it at the same distance. Their plane is inclined to the ecliptic in an angle of 310. In consequence of this obliquity of position, they al ways appear elliptical to us, but with an eccentricity so variable as to ap pear, occasionally, like a straight line drawn across the planet; in which case they are visible only by the aid of superior instruments. Such was their position in April, 1833; for the Sun was then passing from their south to their north side. The rings intersect the ecliptic in two opposite points, SATURN'S RINGS.

Why should we judge, previous to observation, that these rings must revolve «round him ? Does observation confirm this opinion? In what time do the rings revolve about the planet? Is this a greater or less time than a satellite at the same dis tance would require to revolve about it? Why do the rings always appear elliptica. to us? To what extent does the eccentricity of the rings vary? What is the posttion of the rings with regard to the ecliptic?

which may be called their nodes. These points are in longitude 1730, and 350 degrees. When, therefore, Saturn is in either of these points, his ringa will be invisible to us. On the contrary, when his longitude is 80o, or 1⁄2а the rings may be seen to the greatest advantage. As the edges of the rings will present themselves to the Sun twice in each revolution of the planet, it la obvious that the disappearance of them will occur once in about 15 years; subject, however, to the variation dependent on the position of the Earth at that time.

The preceding diagrams are a very good representation of the form and position of the rings as they appear to a spectator during one complete revolution of Saturn through the signs of the ecliptic.

By reference to the figure, it will be seen, that when Saturn is in either of the first six signs, the Sun shines on the south side of the rings; and that while he is in either of the last six signs, upon their north side.

The following are the dates during the ensuing revolutions of the planet, when its mean heliocentric longitude is such that the rings will (if the Earth be favourably situated) either be invisible, or seen to the greatest advantage.

The distance between Saturn and his inner ring, is only 21,000 miles; being less than a tenth part of the distance of our Moon from the Earth. The breadth of the dark band, or the interval between the rings, is hardly 3,000 miles.The breadth of the inner ring is 20,000 miles. Being only about the same distance from Saturn, it will present to his inhabitants a luminous zone, arching the whole concave vault from one hemisphere to the other with a broad girdle of light.

The most obvious use of this double ring is, to reflect light upon the planet in the absence of the Sun; what other purposes it may be intended to subserve, is to us unknown. The sun, as has been shown, illuminates one side of it during 15 years, or one half of the period of the planet's revolution and, during the next 15 years, the other side is enlightened in its turn.

;

Twice in the course of 30 years, there is a short interval of time when neither side is enlightened, and when, of course it ceases to be visible;—namely, at the time when the Sun ceases to shine on one side, and is about to shine on the

What is the longitude of these nodes? In what position of Saturn, then, will the rings be invisible to us, and in what position will they be seen to the best advantage? How often will the disappearance of the rings occur? Explain this. In what signs will the planet be when the Sun shines on the south side of the rings, and in what on the north side? What is the distance between Saturn and his inner ring? How great is this, compared with the distance of our Moon from the Earth? What is the distance between the two rings? What is the breadth of the inner ring? What must be its appearance at Saturn? What is the most obvious use of this double ring? How long a time does the Sun enlighten each side of it alternately? How often, and in what circumstanses, is neither side enlightened, and the ring, of course, invisible?